Building home on pre-existing foundation??

I'm planning to build a home on this pre-existing foundation that was poured back in 2005. The prior owner got sick and abandoned the project. The lot is located in northern Utah at about 6,800 feet, lots of snow, freezing temps, very high water table in the spring, there is a substantial creek that runs close to the foundation. Every year the foundation fills up with water when the snow melts. Recently I pumped out the water, and it didn't fill back in, so I think the water table is below the dirt on the inside of the foundation, which is a couple feet below grade. I recently back- filled around the outside of the foundation, but for the twelve years it was not backfilled. So every year the footings were exposed to lots of water and freeze / thaw cycles. I had an engineer inspect the foundation and he thinks its ok to build on, he did an elevation analysis with surveying equipment and its all within an inch. There are hairline cracks, I was told those are normal. Also had a couple seasoned home builders look at it, and they think its ok to build on, not ideal, but sufficient for a single story home.

Does anyone have any advice for me on this? I'm worried mostly about the soundness of the footings, with all the water and freezing and thawing. Or considering there are no major cracks and its level, maybe its ok? Soil test? Have someone analyse a core sample? Any advice related to foundation or otherwise would be much appreciated!

Currently getting house plans wrapped up, hoping to apply for building permit in the next few weeks.

Nice outdoor pool!
Joe, this is a UK site but here goes with some info. Do the existing foundations go below the frost line, they should. When the foundations were checked by your engineer did he suggest how strong they were, could you build a 2 storey house? Will the house be a typical US build, mainly wood? if so it will be light and won't overload the foundations. Will there be a basement? is the concrete complete, having a floor?
From here it looks OK and good grade concrete, I would build a wooden house on it. But I am a UK electrician, but I have built my own house in Shropshire UK.

Bob, thanks for the feedback! Yes, I'm definitely targeting a light-weight wood frame home, single story with a loft. The frost line is 36" here, it does appear the footings would go down at least that far. They are 24" wide 10" deep I believe. I think I should get a soil test done, so I'll dig down in a spot near the footing and hire a geo-technical engineer to test the soil density right next to the footing. I think that would be good information, considering how water logged this area gets in the spring.